To calculate the average atomic weight, each exact atomic weight is multiplied by its percent abundance, then, add the results together. If the natural abundance of 63Cu is assigned x, the natural abundance of 65Cu is 1-x (the two abundance always add up to 1). So the solution is: (63)(x)+(65)(1-x) = 63.55, 63x+65-65x=63.55, x=0.725=72.5%. The natural abundance of 63Cu is 72.5%, that of 65Cu is 1-72.5%=27.5%.
